 Re: Furuno 582L
Actually if you look closley its usually more like 2 or 3 feet. It comes from a bunch of sources so you might as well know about some of them.
The first thing is that the sonar has some minimum distance at which it can no longer seen anything at all. That is because the way the work is they make a sound and then they sit and listen for the echo for moment. However it takes a little bit of time to make the sound and anything that is very close will actually return an echo before the listening side of the transucer has begun to work. Its all in the timing. For most fish finders that minimum distance is somewhere between 2 and 4 feet, anything closer is just red (or dark gray). Nothing you can do with the controls can adjust for this part of the red band
The other thing that causes that top rim of red is simple turbulence and to a small extent air bubbles created by the hull passing through the water. In essence the boat creats a small layer around itself into which it can not see but it can see beyond. You can do something about this component of the red band. In order to see things like distrubances in the water the fish finder's sensitivity has to be turned way up. This is what Gain really refers to. I have met folks who thought increases in the Gain increased the power the fish finder was putting out. That is not the case. Gain refers to the sensitivity of the receiver and so turning it up means that you are adjusting the receiver to its maximum sensitivity. That is why turning the gain down will remove part of the red band. What it is doing is essentially ignoring it by not listening closley enough to hear it. That works of course but the relaxed sensitivity extends below the red band as well so what you are doing in effect is loosing efficiency in the area where you can see in order to remove a distraction in the area where you can't see anyway. Perplexing problem.
If you run in shallow water and are using the fish finder to track bottom and keep you from parking the boat in the sand at speed then certainly turn the gain down to get rid of the red at the top. The reason is that you need to see the red at the bottom and so by removing the band at the top it makes your task easier. However if you're in water shallow enough that the band is distracting (its actually always there) at your range scale but you're fishing go ahead and live with the red because the increased gain will still let you spot fish better.
Just opnion here, by the way. Don't mistake it for a lesson in fish finder use. Its just how one old misguided soul thinks they work best. The world is full of people who know one hell of a lot more about fish finders and how to use them than I do.
Thom
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